This invention generally relates to radio systems and, more specifically, to a precision timing generator for impulse radio technologies, such as communication systems, radar, and security systems.
Recent advances in communications technology have enabled communication systems to provide ultra-wideband communication systems. Among the numerous benefits of ultra-wideband communication systems are increased channelization, resistance to jamming and low probability of detection.
The benefits of ultra-wideband systems have been demonstrated in part by an emerging, revolutionary ultra-wideband technology called impulse radio communications systems (hereinafter called impulse radio). Impulse radio was first fully described in a series of patents, including U.S. Pat. No. 4,641,317 (issued Feb. 3, 1987), U.S. Pat. No. 4,813,057 (issued Mar. 14, 1989), U.S. Pat. No. 4,979,186 (issued Dec. 18, 1990), U.S. Pat. No. 5,363,108 (issued Nov. 8, 1994) and U.S. Pat. No. 4,743,906 (issued May 10, 1988) all to Larry W. Fullerton. A second generation of impulse radio patents includes U.S. Pat. No. 5,677,927 (issued Oct. 14, 1997), U.S. Pat. No. 5,687,169 (issued Nov. 11, 1997) and co-pending application No. 08/761,602 (filed Dec. 6, 1996; now allowed) to Fullerton et al. These patent documents are incorporated herein by reference.
Basic impulse radio transmitters emit short Gaussian monocycle pulses with tightly controlled pulse-to-pulse intervals. Impulse radio systems use pulse position modulation, which is a farm of time modulation in which the value of each instantaneous sample of a modulating signal is caused to modulate the position of a pulse in time.
For impulse radio communications, the pulse-to-pulse interval is varied on a pulse-by-pulse basis by two components: an information component and a pseudo-random (PN) code component. Generally, spread spectrum systems make use of PN codes to spread the information signal over a significantly wider band of frequencies. A spread spectrum receiver correlates these signals to retrieve the original information signal. Unlike spread spectrum systems, the PN code for impulse radio communications is not necessary for energy spreading because the monocycle pulses themselves have an inherently wide bandwidth. Instead, the pseudo-random code of an impulse radio system is used for channelization, energy smoothing in the frequency domain, and jamming resistance (interference rejection.)
Generally speaking, an impulse radio receiver is a homodyne receiver with a cross correlator front end. The front end coherently converts an electromagnetic pulse train of monocycle pulses to a baseband signal in a single stage. The data rate of the impulse radio transmission is typically a fraction of the periodic timing signal used as a time base. Each data bit time position usually modulates many of the transmitted pulses. This yields a modulated, coded timing signal that comprises a train of identically shaped pulses for each single data bit. The cross correlator of the impulse radio receiver integrates multiple pulses to recover the transmitted information.
In an impulse radio communication system, information is typically modulated by pulse-position modulation. That is, the time at which each pulse is transmitted is varied slightly from the predetermined pulse-to-pulse interval time. One factor limiting the effectiveness of the communication channel is the accuracy with which the pulses can be positioned. More accurate positioning of pulses can allow the communication engineer to achieve enhanced utilization of the communication channel.
For radar position determination and motion sensors, including impulse radio radar systems, precise pulse positioning is crucial to achieving high accuracy and resolution. Limitations in resolution of existing systems are partially a result of the limitations in the ability to encode a transmitted signal with a precisely timed sequence. Therefore, enhancements to the precision with which timing signals can be produced can result in a higher-resolution position and motion sensing system.
Impulse radio communications and radar are but two examples of technologies that would benefit from a precise timing generator. A high-precision timing generator would also find application in any system where precise positioning of a timing signal is required.
Generating such high precision pulses, however, is quite difficult. In general, high precision time bases are needed to create pulses of short duration having tightly controlled pulse-to-pulse intervals. Currently available analog or digital integrated circuit timers are not capable of creating such high precision pulses. Typical impulse radio timer systems are relatively complex, expensive, board level devices that are difficult to produce. A small, low power, easily produced, timer device would enable many new impulse radio-based products and bring their advantages to the end users.
This invention contemplates precision timing generators and associated methods that overcome the problems present in the prior art. One aspect of the invention relates to precision timing generator apparatus. In one embodiment, a precision timing generator according to the invention includes a coarse timing generator that generates a coarse timing signal from a clock signal and a timing command input.
The precision timing generator according to the invention also includes a fine timing generator. The fine timing generator has a sinusoidal-signal generator that receives the clock signal and derives a sinusoidal signal from the clock signal. The fine timing generator further includes a phase shifter that receives the sinusoidal signal and the timing command input and shifts the phase of the sinusoidal signal based on the timing input to generate a phase shifted sinusoidal signal. The phase shifted sinusoidal signal has a desired phase shift. A combiner provides a timing signal by combining the coarse timing signal and a fine timing signal derived from the phase shifted sinusoidal signal.
More particularly, in exemplary embodiments, the phase shifter includes a filter, a first multiplier, and a second multiplier. The filter receives the sinusoidal signal and outputs an in-phase signal and a quadrature signal. The filter has a first filter section that receives the sinusoidal signal and outputs a filtered sinusoidal signal, and a second filter section that receives the filtered sinusoidal signal and outputs the in-phase signal and the quadrature signal.
The first multiplier multiplies a cosine of the desired phase shift with a signal derived from the in-phase signal and outputs a first product signal. The second multiplier multiplies a sine of the desired phase shift with a signal derived from the quadrature signal and outputs a second product signal.
The first multiplier has a first current-steering circuit that receives the signal derived from the in-phase signal and steers a first current and a second current to produce the first product signal. The first multiplier also includes a second current-steering circuit that receives the cosine of the desired phase shift and provides the first current and the second current. The second multiplier has a third current-steering circuit that receives the signal derived from the quadrature signal and steers a third current and a fourth current to produce the second product signal. The second multiplier also includes a fourth current-steering circuit that receives the sine of the desired phase shift and provides the third current and the fourth current.
The phase shifter further includes a summer that adds the first product signal to the second product signal and outputs a sum signal, and a filter that receives the sum signal and filters the sum signal to provide the phase shifted sinusoidal signal.
Another aspect of the invention relates to methods for generating precision timing signals. In one embodiment, a method according to the invention includes generating a coarse timing signal from a clock signal and a timing command input, and deriving a sinusoidal signal from the clock signal. The method also includes shifting the phase of the sinusoidal signal based on the timing command input to generate a phase shifted sinusoidal signal that has a desired phase shift. Finally, the method combines the coarse timing signal and a fine timing signal derived from the phase shifted sinusoidal signal to provide the precision timing signal.
More particularly, in exemplary embodiments, shifting the phase of the sinusoidal signal further includes filtering the sinusoidal signal to generate an in-phase signal and a quadrature signal. Filtering the sinusoidal signal results in generating a filtered sinusoidal signal and deriving the in-phase signal and the quadrature signal from the filtered sinusoidal signal.
Shifting the phase of the sinusoidal signal also includes multiplying a cosine of the desired phase shift with a signal derived from the in-phase signal to generate a first product signal, and multiplying a sine of the desired phase shift with a signal derived from the quadrature signal to generate a second product signal.
Multiplying a cosine of the desired phase shift and the signal derived from the in-phase signal includes providing a first current and a second current based, at least in part, on the cosine of the desired phase shift, and steering the first current and the second current based, at least in part, on the signal derived from the in-phase signal to provide the first product signal. Similarly, multiplying a sine of the desired phase shift and the signal derived from the quadrature signal includes providing a third current and a fourth current based, at least in part, on the sine of the desired phase shift, and steering the third current and the fourth current based, at least in part, on the signal derived from the quadrature signal to provide the second product signal. Furthermore, shifting the phase of the sinusoidal signal also includes adding the first product signal to the second product signal to provide a sum signal, and filtering the sum signal to generate the filtered sinusoidal signal.